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Bureau of Mines Information Circular/1982 




Some Design Factors for Windows 
and Lenses Used in Explosion- Proof 
Enclosures 



By Lawrence W. Scott 




UNITED STATES DEPARTMENT OF THE INTERIOR 






Wm 



,4eJ SUU • &ifea^ of f*"" ' ^ 



Information Circular 8880 



Some Design Factors for Windows 
and Lenses Used in Explosion- Proof 
Enclosures 

By Lawrence W. Sott , c fc-on 




UNITED STATES DEPARTMENT OF THE INTERIOR 
James G. Watt, Secretary 

BUREAU OF MINES 
Robert C. Norton, Director 







This publication has been cataloged as follows: 



Scott, Lawrence W 
















Some design factors 


for w 


indows 


and lenses used 


in 


explosion-proof enclosures. 


% 












(Information circular / U.S 


Bureau of W. 


ines 


; 8880) 






Supt. of Docs, no.: 


I 28.27:8880. 












1. Mine lighting- 


-Safety 


measures. 


2. 


Explosions— Safety mea- 1 


sures. I. Title. II. 


Series: 


Information 


circular (United States. 


Bu- 


reau of Mines) ; 8880. 
















TN295.U4 [TN301] 


622. 


3 [622' 


.8] 


82-600067 


AACR2 





i^ 



CONTENTS 









Page 



^ Abstract 1 

"^ Introduction 2 

^ Materials 3 

Glass 3 

Plastic 3 

Design of glass windows 4 

Design of plastic windows 4 

Sealing and bonding of windows and lenses ;. 6 

Sealants 6 

Adheslves 6 

Surface preparation of adherends 7 

Surface preparation for aluminum 7 

Surface preparation for steel 7 

Surface preparation for brass 8 

Surface preparation for glass 8 

Surface preparation for polycarbonate 8 

Summary 9 



SOME DESIGN FACTORS FOR WINDOWS AND LENSES 
USED IN EXPLOSION-PROOF ENCLOSURES 

By Lawrence W. Scott 1 



ABSTRACT 

This Bureau of Mines report presents several factors that currently 
enter into the design, manufacture, and testing of windows and lenses 
used in explosion-proof enclosures. Emphasis is also given to the seal- 
ing concepts for lenses. Criteria for adhesives and sealants are sug- 
gested based on the survivability of an explosion-proof enclosure as a 
structure, rather than upon the minimum material properties of its con- 
stituents. Procedures for surface preparation of adherends are also 
discussed. 



^Electrical engineer, Pittsburgh Research Center, Bureau of Mines, Pittsburgh, Pa. 



INTRODUCTION 



For the past 5 years, Southwest 
Research Institute, San Antonio, Tex. , 
through funding by the Bureau of Mines 
Coal Mine Health and Safety Program, has 
been analyzing several of the critical 
factors involved in the design of 
explosion-proof enclosures. 2 These 
include safety factors, weld quality 
standards, quality assurance standards, 
and reliability of enclosures with win- 
dows. The progress presented here is 
expected to be of considerable interest 
to the mining industry in general and to 
designers of explosion-proof enclosures 
in particular. 



must be certain that the window or lens 
is adequate for the design conditions of 
the enclosure, particularly the dynamic 
pressure, temperature, point impact, 
thermal shock, and corrosive effect of 
the operational environment. 

The windows and lenses discussed in 
this report are intended for use only in 
enclosures with window service conditions 
defined by 

Maximum dynamic pressure (Pj^ gen- 
erated by an explosion of methane-air 
inside the enclosure. 



Title 30 of the Code of Federal Reg- 
ulations (CFR), Part 18.2, defines an 
explosion-proof enclosure as 



Design pressure (P) equal to P^j 
X 1.5 or 150 psig (1,034 KPa) , whichever 
is greater. 



" an enclosure that . . . 

is so constructed that it will 
withstand internal explosions 
of methane-air mixtures: (1) 
without damage to or excessive 
distortion of its walls and 
cover(s), and (2) without igni- 
tion of surrounding methane-air 
mixtures or discharge of flame 
from inside to outside the 
enclosure. " 

Several different types of electri- 
cal equipment fall under this definition, 
including power enclosures, distribution 
boxes, splice boxes, and ballast boxes. 
This report is concerned primarily with 
power enclosures and luminaires, (light- 
ing fixtures mounted on coal mining 
machinery). The windows and lenses built 
into luminaires are often fixed in place 
with adhesives and sealants and are gen- 
erally subjected to more severe thermal 
environments than are other explosion- 
proof enclosures. 

Windows in explosion-proof enclo- 
sures require careful design, fabrica- 
tion, and installation. The designer 

^USBM contract H0377052, Analysis of 
Schedule 2G Enclosures; Bureau of Mines 
Technical Project Officer/ Lawrence W. 
Scott. 



Maximum temperature (T^j) generated 
by internal light source, equal to design 
temperature. 

Pressure cycles generated by re- 
peated explosions at design pressure and 
temperature. 

Thermal shock generated by immersion 
of window at design temperature to water 
having a temperature between 59° F 
(15° C) and 68° (20° C). 

Physical shock generated by point 
impact of an object with 8 ft-lb kinetic 
energy at the center of the window. 

The windows and lenses discussed are 
subject to the following restrictions: 

1. The operating temperature shall 
not exceed 302° F (150° C) on the exte- 
rior of the window. 

2. The fluids contacting the sur- 
face of the window shall be only those 
typically found in an underground mine 
environment (that is, humid air, mine 
water, and lubricating and hydraulic oils 
used on mechanized equipment) . 

3. The total number of pressure 
cycles during the operational life of the 
enclosure shall not exceed 1,000. 



MATERIALS 



Windows and lenses should be fabri- 
cated only from materials suited for the 
operational environment encountered in 
mines. The suitability of a material is 
based either on documented extensive past 
experience, or on exhaustive evaluation 
by a materials testing laboratory in sim- 
ulated mine environments. At present, 
glass and polycarbonate plastics are con- 
sidered practical materials for fabrica- 
tion of windows and lenses. 

Glass 

The chemical composition, casting 
process, and thermal treatment determine 
the physical, chemical, optical, and 
electrical properties of glass. Because 
of a very complex relationship among 
these variables, no single glass composi- 
tion, casting process, or thermal treat- 
ment is considered superior to others. 
Thus, the designer is free to select the 
combination of fabrication parameters 
that best matches a specific set of prod- 
uct requirements. 

The primary advantages of glass are 
its ability to retain its physical and 
optical properties under high ambient 
temperature, ultraviolet radiation, and 
humidity for a long period of time; to 
resist surface abrasion by rock parti- 
cles; and to tolerate immersion in aque- 
ous and organic solvents without initi- 
ation of stress cracking or corrosion. 
Glass windows can tolerate 100-percent 
relative himiidity, temperature of 400° F 
(204° C), intense ultraviolet radiation, 
and continuous or intermittent immersion 
in basic or acidic water or organic sol- 
vents for indefinite periods. 



contact is usually accomplished by 
inserting gaskets between the glass and 
metallic components of the seat assembly. 
Protection against breakage by impact is 
generally provided by an external shield 
in the form of a cage or plastic enve- 
lope, or by precompressing the glass win- 
dow surfaces with thermal tempering or 
chemical ion exchange. 

Because of their history of success- 
ful use in enclosures, the following 
glasses are practical for use as windows 
in enclosures: (1) Borosilicate glasses, 
(2) soda lime glasses, and (3) silica 
glasses. 

Plastic 

The high temperature, hxomidity, 
intensive ultraviolet radiation, and 
presence of vapors from petroleum-based 
oils tend to degrade rapidly the mechani- 
cal properties of plastic windows and 
lenses in explosion-proof enclosures. 
Some plastics deteriorate in lamp enclo- 
sure service faster than others, but even 
the most resistant ones age sufficiently 
to mandate their removal from service in 
less than 10 years. For this reason, a 
thorough engineering evaluation of plas- 
tic material must be conducted prior to 
its selection for service as a window in 
an explosion-proof enclosure. At pres- 
ent, polycarbonate plastic is considered 
practical for fabrication of windows and 
lenses for enclosures; however, Mine 
Safety and Health Administration (MSHA) 
policy limits the design temperature to 
240° F (115° C). Industrial experience 
has shown that three other restrictions 
should be noted: 



The primary shortcomings of glass 
are its brittleness and low tensile 
strength. To compensate for these short- 
comings, the design of the window seat 
assembly must provide, whenever feasible, 
protection against point contact with the 
metallic components of the enclosure and 
impact by rock fragments capable of frac- 
turing the window. The protection 
against fracture initiated by point 



1. Contact with hydraulic oil and 
petroleimi-based lubricants prohibited. 

2. Low-ultraviolet (UV) environment 
(no more than 20-percent loss in lumin- 
aire output after one year of continuous 
operation) . 

3. Service life < 4 years. 



DESIGN OF GLASS WINDOWS 



The thickness of a glass window 
should be based primarily on the design 
pressure and only secondarily on the 
design impact resistance. The design 
pressure of the window should equal or 
exceed 150 psig or 1.5 times the maximum 
dynamic pressure generated by a methane- 
air explosion inside the enclosure, 
whichever is greater. For safety pur- 
poses, the calculated value of nominal 
tensile stress in the window at design 
pressure should not exceed 1,000 psig for 
annealed glass and 2,000 psig for tem- 
pered glass. 

Windows may be fabricated by any 
acceptable commercial technique. Typical 
techniques are (1) cutting and grinding 
from plate stock or (2) pressing and 
grinding. The high- and low-pressure 
faces of the windows, except the bearing 
surface, should be polished and free from 
scratches and pits. Molds used for mold- 
ing should have their surfaces polished. 
The bearing surfaces on plane, cylindri- 
cal, and dome windows should be flat 
within 0.010 inch. Ground surfaces 
should be finished with 220-micrometer or 
finer abrasive powder. The ground bear- 
ing surfaces of a pressed plane window 
should be located above the polished sur- 
face of the window to minimize the stress 
rise at the boundary line between the 
ground and polished surfaces. All sharp 
corners should be chamfered and free of 
chips exceeding 0.020 inch in size. 

After grinding and polishing, the 
windows should be subjected to a thermal 
treatment. The thermal treatment for 
windows designed to serve in the annealed 
state should consist of subjecting them 
to the appropriate annealing procedure 
for that glass composition. The anneal- 
ing procedure should be capable of 



decreasing the residual stress in the 
window below 600 psi. The thermal treat- 
ment for windows designed to serve in the 
tempered state should consist of subject- 
ing them to the appropriate tempering 
treatment for that glass composition. 
The selected tempering procedure should 
generate residual compressive stresses 
whose magnitude on the surface of the 
window exceeds 4,000 psi. 

Unless information is available 
beforehand on the magnitude of the 
dynamic peak pressure generated by a gas 
explosion inside a selected enclosure 
design, the following design procedure 
should be followed: 

1. Design the window assembly for 
150-psig service. 

2. Fabricate the window and window 
seat assembly, install them in the enclo- 
sure, and record the maximum dynamic 
pressure generated inside the enclosure 
by a methane-air explosion according to 
CFR 30, Part 18.62. 

3. If the recorded pressure is less 
than 100 psig, the original design pres- 
sure of 150 psig is considered satisfac- 
tory. If the dynamic pressure exceeds 
100 psig, the window must be redesigned 
to a new design pressure equal to 1.5 
times the measured dynamic pressure. 

4. If the window is to be used in 
a luminaire, an operational lamp test 
must be conducted to determine whether 
the temperature of the external window 
surface exceeds 302° F (150° C) when the 
enclosure is located in an ambient envir- 
onment at 100-percent relative humidity 
and 100° F temperature. 



DESIGN OF PLASTIC WINDOWS 



The thickness of a plastic window 
should be based primarily on the design 
pressure and temperature, and secondarily 
on the design impact resistance. The 
design pressure of the window should 



equal or exceed 150 psig or 1.5 times the 
dynamic pressure generated by a methane- 
air explosion inside the enclosure, 
whichever is greater. The calculated 
values of maximum nominal tensile and 



compressive stresses in the window at 
design pressure should not exceed the 
nominal stress values for the selected 
plastic material composition and design 
temperature. At present, only polycar- 
bonate plastics are qualified for window 
service. Maximum design values for this 
plastic are — 

Grade — UV stabilized, clear 
Temperature~240° F (115° C) 
Tensile stress — 1,100 psi 
Sheer stress — 1,000 psi 
Compressive stress — 1,500 psi 

The window may be fabricated by (1) 
machining from plate or bar stock, (2) 
molding, or (3) extruding. Molded and 
extruded windows may require some machin- 
ing to bring them in conformance with 
specified dimensions and bearing surface 
finishes. The high- and low-pressure 
faces of windows should have a finish of 
32 rms3 or finer. 

During the fabrication process, sub- 
stances and fluids detrimental to plas- 
tics should be avoided. Windows fabri- 
cated by machining flat or bar stock 
should be annealed twice, once after 
rough machining and again when the window 
is completed. Windows fabricated by 
molding or extrusion should be annealed 
at least once, when all manufacturing 
operations have been completed. The 
annealing procedure must follow the 

^rms (root mean square) is a surface 
roughness term, referring to relatively 
fine spaced surface irregularities, the 
height, width, and direction of which 
establish the predominant surface pat- 
tern. The rms average is obtained by 
squaring the height measurements of the 
peaks and valleys, taking the average of 
the squared values, and then extracting 
the square root of this average. 



recommendations of the plastic supplier; 
however, the annealing temperature mast 
exceed the maximum design temperature of 
the given plastic by at least 10° F. 

Dimensions of finished plane windows 
with square edges should be within 
±0.032 inch of nominal dimensions. The 
major diameter (or width and height) of 
finished plane and spherical windows with 
inclined bearing surfaces should be 
within ±0.020 inch of nominal dimensions. 
The thickness of finished windows should 
be within ±0.032 inch of specified nom- 
inal thickness. The diameter and wall 
thickness of cylindrical windows should 
also be within ±0.032 inch of specified 
nominal dimensions. 

If either the peak pressure or win- 
dow surface temperature is unknown, the 
designer should (1) design the window for 
150 psig pressure and maximum allowable 
temperature for the plastic material com- 
position, and (2) experimentally confirm 
the design values. The experimental 
confirmation should be conducted on an 
operational enclosure dedicated to the 
selected window design by — 

1. Performing an internal explo- 
sion test according to Schedule 2G, 
Part 18.62 and recording the maximum 
internal dynamic pressure. 

2. In the case of luminaires, per- 
forming on operational test by turning 
the lamp on inside the enclosure and 
recording the interior surface tempera- 
ture of the window after 8 hours of con- 
tinuous operation. If the recorded 
dynamic peak pressure multiplied by 1.5 
is below 150 psig and the recorded sur- 
face temperature is below or equal to the 
maximum design temperature, the design is 
considered satisfactory. 



SEALING AND BONDING OF WINDOWS AND LENSES 



The windows and lenses used in 
explosion-proof enclosures, especially 
luminaires, are often held in place by 
sealants and adhesives and secured with a 
mechanical attachment. 

There are two broad categories into 
which materials that are candidates for 
use in explosion-proof enclosures may 
fall. The first of these is use as a 
sealant , where the product is to be used 
in such a way that it is not required to 
support or transmit any significant 
stresses or loads; for example, a product 
used as a barrier to protect against 
water seepage into the enclosure from 
around a lens, where the lens is backed 
up by a retaining ring. Because they 
must maintain contact with their sub- 
strate surfaces, under conditions of 
expansion and contraction, sealants must 
possess some adhesive capacity. Sealants 
are normally soft, compliant materials 
that may swell or shrink to accommodate 
environmentally induced forces (hygro- 
thermal effects) and that possess adhe- 
sive characteristics only to the extent 
that the bondline is maintained intact 
under action of these secondary forces. 

The second use category is that of 
an adhesive . An adhesive is considered 
to be a material used to bond two materi- 
als together and is capable of reacting 
and transmitting structural and secondary 
(environmental) forces imposed upon it 
during equipment operation. An adhesive 
may have all the characteristics: of a 
sealant, but it is distinguished from a 
sealant in that the lens or window is 
held in place primarily with the adhe- 
sive. Although adhesives should be more 
compliant (of lower modulus) than the 
substrate to which they adhere, they have 
much higher adhesive and cohesive 
strength properties than sealants. 

Sealants 

Most sealant materials for use in 
explosion-proof enclosures are single- 
component, room-temperature vulcanizing 



silicones, commonly called RTV's. RTV's 
are widely used for the following 
reasons: 

1. The elastomeric nature of RTV 
silicones gives them the compliant qual- 
ities required to accommodate dimensional 
changes due to differences in thermal 
expansion coefficients between the lens 
and its housing. 

2. RTV silicones are considerably 
more resistant to the effects of tem- 
perature than are organic sealants and 
adhesives. They generally retain their 
properties relatively well up to about 
200° C (392° F). 

3. The elastomeric nature of cured 
RTV's provides some measure of shock pro- 
tection for the lens. 

4. These silicones have good chemi- 
cal resistance in that they tend not to 
be affected by moisture or weak acids and 
bases. 

5. Although their strength and 
adhesion properties are below those of 
most organic sealants, RTV silicones are 
adequate for many properly designed 
joints, and their elongation properties 
are generally superior to those of 
organic sealants. 

Adhesives 

At present, practically all adhe- 
sives used for structural purposes in 
explosion-proof enclosures are two- 
component epoxies. In general terms, 
epoxies are either one- or multi- 
component systems, depending upon whether 
the resin and hardener are blended 
together in a single system or stored 
separately. All epoxies are either basic 
resin systems or modified systems. The 
basic materials have no additives and, 
therefore, are in a hard, brittle state. 
Modified systems may have additives, such 
as fillers or other resin alloys, or may 
have chemical modifications made to the 



resin and/or curing agent. Most one- 
component epoxies require elevated- 
temperature curing, and all two- 
component epoxies require careful mixing 
of the monomer and hardener. Epoxy 
adhesives have good high temperature per- 
formance and low shrinkage, but are very 
sensitive to formulation and application 
procedures. 

Where a design calls for a true 
adhesive, several features aside from 



adhesive strength should be considered 
in selecting the adhesive. Among 
these are (1) long-term tolerance to 
environmental factors without becoming 
brittle, (2) minimum shrinkage due to 
natural aging, (3) permanent barrier 
protection against water infiltration, 
and (4) acceptable level of emis- 
sion of combustible decomposition 
products. 



SURFACE PREPARATION OF ADHERENDS 



In general, the surface preparation 
of an adherend is the same regardless of 
the adhesive used. Differences arise 
when a primer, a coupling agent, or an 
adhesion promoter is to be used. In such 
cases, a match must be made of the inter- 
facial agent to the adherend and the 
adhesive. 

There are as many bonding procedures 
as there are lens-substrate combinations 
and applicable adhesives on the commer- 
cial market. Because of this variety, 
recommendations of the adhesive supplier 
with regard to surface preparation should 
be followed scrupulously. In addition, 
the following general procedures for 
substrates found in explosion-proof 
enclosures, such as aluminum, steel, 
brass, glass, and polycarbonate, are 
recommended. 

Surface Preparation for Aluminum 

Although both chemical cleaning 
(etching) and mechanical abrasion (fine 
abrasive) are possible, etching results 
in higher reliability. An etching method 
that has been found to be satisfactory 
with both epoxy and urethane adhesives 
follows: 

1. Degrease in a vapor bath of tri- 
chloroethylene (TCE). 

2. Etch for 20 minutes at 66° C 
(150° F) with a fresh solution of 
65.4 weight-percent water, 26.9 weight- 
percent sulfuric acid, and 7.7 weight- 
percent sodium dichromate dihydrate 



(Na2Cr207 2H2O). Note that a fresh solu- 
tion is necessary. 

3. Wash with distilled water, 

4. Dry at 66° C for 10 minutes. 

5. Unprimed parts should be used 
within 3 hours. 

Surface Preparation for Steel 

Because of the wide variation in 
steel compositions, this adherend can be 
a special problem. For example, mild 
steels respond nicely to 
abrasion, while stainless 
degreasing, cleaning with 
an acid etch. 



degreasing and 
steels require 
detergent, and 



Perhaps the two most common problems 
are corrosion on the surface prior to 
bonding and disruption of bonding due to 
water ingression to the interface. This 
means that although adhesion may be good 
initially, it can deteriorate rapidly in 
use. 

Surface preparation for steels, 
then, is a critical factor. Also, prim- 
ers or adhesion promoters find greater 
application here than on other metallic 
substrates. 

The following procedure omits acid 
etch, rinse, and dry steps, which are not 
necessary on mild steel: 

1. Wipe and vapor-degrease with TCE 
or perchloroethylene (PCE). 



2. Grit blast. 

3. Degrease again. 

4. Dry. 

5. Use immediately after drying. 

Surface Preparation for Brass 

Brass and other copper alloys pose a 
problem in good adhesive bonding owing to 
the rapid formation of oxide coatings. 
Although there is a commercial product 
that intentionally produces a tightly 
adhering black, oxide coating to which the 
adhesive forms a bond (Ebonol C Special, 
Enthane Co., New Haven, Conn.). 4 

Several acid etchants are easy to 
produce from commonly available materi- 
als. A process using one of these 
follows: 

1. Vapor-degrease. 

2. Etch for 1 to 2 minutes in 
a solution of 50 weight-percent concen- 
trated hydrochloric acid, 20 weight- 
percent ferric chloride (FeCl3), and 
30 weight-percent water. 

3. Rinse with distilled water, dry, 
and use as soon as possible. 

Surface Preparation for Glass 

Generally speaking, glass provides a 
good bond capability with many sealant 
and adhesive materials. Excellent adhe- 
sion is afforded by epoxies as well as 
acrylics, unsaturated polyesters, and 
polyvinyl butyral. Cleanliness is the 
most important factor in bonding to 
glass. Abrasion (fine-grit blasting or 
#400 grit paper) can be a supplement. If 
a sizing is present on the surface, such 

^Use of company and brand names is for 
identification purposes only and does not 
imply endorsement by the Bureau of Mines. 



as in glass fibers or cloth, it must be 
removed by a heat treatment at 450° C for 
24 hours. A general procedure follows: 

1. Clean with a solvent (alcohol, 
acetone, or TCE). 

2. Dry and keep dry prior to use. 

Coupling agents have found great 
utility in forming adhesive bonds to 
glass and will improve reliability of the 
bond. Various coupling agents are com- 
mercially available to enhance adhesion 
with both thermoplastic and thermosetting 
resins. Note that the adhesive for glass 
should not embrittle with age because of 
thermal expansion of the substrates. 

Surface Preparation for Polycarbonate 

Adhesives that cure at room tempera- 
ture are preferred when bonding polycar- 
bonate to metals, owing to the differ- 
ences in thermal expansion of the two 
substrates. Most adhesives tend to 
embrittle polycarbonate lenses with time, 
and for this reason mechanical attachment 
(such as threading the ends of a polycar- 
bonate tube in the case of fluorescent 
luminaires) or solvent-bonding systems 
are preferable. However, when adhesive 
bonding is indicated, the adhesive choice 
should be made with full regard to 
the temperature and water chemistry 
environments to which the adhesive will 
be exposed. Both cleaning and abrasion 
are recommended for polycarbonate sur- 
faces, as follows: 

1. Wipe clean with alcohol or a 
hydrocarbon (hexane, heptane, naphtha, or 
toluene) . 

2. Abrade with 200-grit sandpaper. 

3. Scrub with abrasive cleanser. 

4. Rinse with alcohol, rinse again 
with distilled water, and dry. 



SUMMARY 



Windows and lenses for use in 
explosion-proof enclosures require care- 
ful design, fabrication, and installa- 
tion. The designer must be sure that the 
window or lens is adequate for its 
intended use and will tolerate an under- 
ground environment. 

Although mechanical attachment of 
windows and lenses is preferred, adhe- 
sives and sealants are often used to 
secure windows in enclosures. Care must 
be taken when selecting an adhesive or 
sealant to insure that it can withstand 



the environmental factors encountered in 
mines. Special attention should be given 
to the preparation of adherends to insure 
that a proper bond is obtained. 

Finally, the design and testing sug- 
gestions discussed in this report are not 
all encompassing. The designer of win- 
dows, lenses, and enclosures should be 
cognizant of Schedule 2G of the CFR, 
which details performance of windows and 
lenses in such tests as the impact test 
and thermal shock test. 



INT. -BU. O F MINES, PGH., PA. 26078 



4562 



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